FIG. 15 illustrates a hitherto widely employed control for speed changing with a transmission. In the illustrated case, speed changing from a first speed to a second speed is taken into account. This conventional control for speed changing is such that a clutch for the first speed is turned off when a command for speed changing is outputted and at the same time a hydraulic oil starts to flow in a clutch for the second speed. Consequently, a hydraulic pressure in the clutch for the first speed is lowered from a predetermined pressure to a level of zero when the command for speed changing is outputted, while a hydraulic pressure in the clutch for the second speed starts to increase gradually after a period of filling time t.sub.f elapses. This filling time t.sub.f designates a period of time required for filling the interior of a clutch pack of the clutch for the second speed with a hydraulic oil.
In this manner, with the conventional apparatus, the time t.sub.f that elapses until the clutch pack is filled with a hydraulic oil is reserved and no torque is transmitted during the period of time t.sub.f. This causes a so-called breathing phenomenon to appear during a period of speed changing with the conventional apparatus and the breathing phenomenon gives rise to a problem concerning running performances such as riding comfortability, acceleration characteristic or the like. Particularly, with respect to a large-sized construction machine, a period of filling time is prolonged due to a large capacity of respective clutches, which has a substantial effect on running performances.
In view of the foregoing problems, in the recent years attention has been paid to a crossover control as disclosed in an official gazette of Japanese Laid-Open Patent NO. 13758/1963 and the crossover control has been lately carried out using electronic type pressure control valves. Specifically, according to the prior invention as proposed by the inventors under Japanese Patent Application NO. 271055/1985, a crossover control as shown in FIG. 16 is carried out using an electronic modulation system for all clutches to prevent an occurrence of breathing during a period of speed changing due to the presence of filling time.
The proposal as shown in FIG. 16 is such that when a command for speed changing is outputted, a hydraulic oil starts to flow in a clutch for the second speed to be next brought in an engaged state, thereafter when the termination of filling is detected, a clutch for the first speed which has been previously brought in an engaged state is turned off and a bulid-up control is initiated for the clutch for the second speed.
According to this proposal, an occurrence of breathing phenomenon due to the presence of filling time t.sub.f is not only prevented reliably but also speed changing can be smoothly achieved, resulting in improved running performances being assured. However, to properly carry out such speed changing, it is required that the filling time t.sub.f is exactly detected and an occurrence of double engagement is prevented without fail.
Hitherto, detection of the filling has been performed by properly controlling a pattern with which a hydraulic pressure is gradually increased as time elapses after a command for speed changing is outputted. Specifically, hitherto, a time required for filling the clutch pack with a hydraulic oil is suitably set on the basis of prior experiments, the termination of filling is identified when the set time elapses after a command for speed changing is outputted, and thereafter a hydraulic pressure is increased gradually.
However, it has been found that detecting of the filling having such time control employed therefor can not absorb fluctuation in filling time. For instance, when the clutch pack is filled with a hydraulic oil before the set time elapses, this allows an useless waiting time to be produced, which brings a factor of causing time lag. In addition, since there is a shortage in reliability of detection, a problem is that the aforementioned time control can not be applied to the conventional crossover control for speed changing.
Additionally, according to this crossover control for speed changing, there is a danger that clutches are brought in double engagement, if releasing and engagement timing of clutches located at the preceding stage as well as at the following stages are deviated from correct ones. Accordingly, this becomes a serious problem from the viewpoint of measures to be taken for safety.
However, any particular measure for assuring safety is not taken with the conventional apparatus and this leads to a fear of causing troubles such as damage or injury of components or instruments.
To prevent an occurrence of double engagement, it suffices that an engaged state of the respective clutches can be determined. To this end, there is a need of watching which clutch is filled with a hydraulic oil having a certain intensity of pressure. To provide structural arrangement for the purpose of watching in that way, there has been made a proposal for arrangement of a strain gauge type pressure sensor or a semiconductor type pressure sensor which is available on the commercial basis. However, the proposal has a problem that the commercial pressure sensor is expensive and it has a difficulty in durability. Further, to prevent an occurrence of double engagement, it suffices that whether or not the clutch is filled with a hydraulic oil having a certain intensity of pressure is known. According, a quantitatively detected value derived by a pressure sensor is not required.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for controlling a hydraulic pressure usable for a clutch which assures that detection of filling and detection of presence or absence of a hydraulic pressure for clutches can be exactly attained by a simple and inexpensive structural arrangement.
Other object of the present invention is to provide an apparatus for controlling a hydraulic pressure usable for a clutch which assures that an overshoot pressure generated on the termination of filling is reduced substantially.
Another object of the present invention is to provide an apparatus for controlling a hydraulic pressure usable for a clutch which assures that a shock appearing during a period of speed changing is reduced by smooth speed changing and an occurrence of double engagement with clutches can be prevented without fail.